Glioblastoma multiforme (GBM), the most common primary malignancy of the brain, has high rates of relapse and mortality despite aggressive treatment with surgery, radiation, and chemotherapy. Improved methods to target and treat GBM are needed, but the blood-brain barrier (BBB) and genetic heterogeneity associated with GBM are obstacles to drug development. The autoimmune disease systemic lupus erythematosus offers an unexpected new approach. We discovered that the lupus anti-DNA autoantibody 3E10 penetrates live cell nuclei and inhibits DNA repair in a manner that does not kill normal cells but is toxic to cancer cells with genetic defects in repair of DNA double- strand breaks (DSBs), including PTEN-deficient cancer cells and tumors. 3E10 penetrates cells via a mechanism that requires both extracellular DNA and the ENT2 nucleoside transporter to be present, which facilitates preferential penetration of 3E10 into tumors due to their expression of ENT2 and increased DNA in their environment. ~40% of primary GBM is PTEN-deficient, and we hypothesize that 3E10 will target GBM, have single agent effects against PTEN-deficient GBM, and can serve as a sensitizing agent for radiation therapy and as a drug delivery ligand for GBM regardless of PTEN status. We have re-engineered a 3E10 fragment, hereafter referred to as Deoxymab-1 (DX1), to maximize effect on cancer cells and minimize risks. In preliminary studies DX1 crosses the BBB to localize into and suppress growth of PTEN-deficient GBM in an orthotopic patient-derived xenograft (PDX) mouse model, and delivers conjugated nanocarriers to orthotopic GBM tumors. We now propose studies to help translate DX1 into a novel therapy for GBM. In Aim 1 we pursue studies to elucidate and enhance the mechanism by which DX1 crosses the BBB in GBM. In Aim 2 the effects of DX1, alone or in combination with radiation therapy, on viability and DNA damage accumulation in GBM and normal cells will be determined. DX1 +/- radiation therapy will then be tested in PDX and syngeneic mouse models of GBM to evaluate efficacy and toxicity. In Aim 3 methods to deliver drug-loaded nanocarriers by surface conjugation with DX1 are developed and tested in orthotopic GBM models. We believe use of a modified nuclear-penetrating lupus anti-DNA autoantibody against GBM is an innovative and compelling new strategy that has potential for significant clinical impact, and the proposed studies will establish a foundation for advancing the DX1 technology to clinical trials.